Yes you can lower 40mm and keep the standard dampers - anymore and the advice is to upgrade.
The rear dampers are a piece of p*ss to change - so you can do them at any time depending upon your budget and/or if your old ones aren't up to the job.
In short - if you have the funds then do a proper job with new springs and dampers all round - if you've haven't got the funds then spings only and replace the dampers later when/if you need to - good luck